This invention relates to the recovery of plural dilute sorbable species from a fluid stream, e.g., the recovery of petroleum fractions such as pentane and hexane from natural gas streams. More particularly, the invention relates to exploiting the physical chemistry insights disclosed in U.S. Pat. No. 4,324,564 in the context of sorption bed systems designed to remove dilute sorbable species from a fluid stream and to recover valuable components of the sorbable species.
Sorbents are widely used for the purification of fluid mixtures. The sorbent material or materials, typically in particulate form or fixed on a support, are contained in a vessel which provides means for passing fluid along a flow path through the interstices among the solids in the bed. A fluid feed stream containing one or more dilute species (sorbates) to be removed, typically at cumulative concentrations no greater than about 15 percent, is introduced into the bed and passed along the flow path in a sorption stage. Inside the bed sorption waves or fronts form which pass along the flow path from a point adjacent the bed entrance in the same direction as the fluid flow, but at a much slower rate.
These sorption fronts are the bed regions wherein changes in sorbent loading and sorbate content in the fluid phase occur. Each front's upstream side is bounded by a bed region characterized by sorbent loadings, sorbate to fluid feed mole ratios, and temperatures characteristic of equilibrium between the sorbent material and the feed. On its downstream side, the sorption front is bounded by a bed region having properties characteristic of equilibrium between the sorbent material and the substantially sorbate-free fluid product. Front boundaries are generally not well defined but rather comprise regions which asymptotically approach equilibrium. As the downstream boundary of the fastest sorption front approaches the bed exit, the concentration of sorbate in the product begins to rise. When the concentration of the sorbate in the product at the exit exceeds some predetermined specification, sorption is discontinued and the bed is regenerated.
In the regeneration stage, a regenerant comprising a hot fluid is passed along the flow path in a cocurrent, or more commonly in recovery operations a countercurrent direction. The high temperature of the regenerant produces a desorption front in the bed which drives sorbate off the surface of the sorbent material and into the flowing regenerant stream. This process continues until the bed is substantially sorbate-free, typically as indicated by the emergence of hot regenerant fluid at the bed exit. The hot, sorbate-free bed is then either cooled or utilized for sorption service while still hot. The introduction of a coolant fluid produces a thermal front which takes heat from the bed.
Sorption bed systems of the type described above are known as "thermal swing" systems because they are regenerated with heat. They have been widely utilized in various industries. For example, natural gas contaminated with water, acid gases, and higher molecular weight hydrocarbons are often treated prior to liquification or delivery to a pipeline to remove the contaminates using a sorption bed system. In situations where the higher molecular weight hydrocarbon content of the gas is high enough to justify economically the capital and energy cost of recovery, such sorption bed systems have been designed to remove the hydrocarbons in a condenser or the like for subsequent separation and purification. Such systems are typically operated with very short sorption and regeneration cycles.
U.S. Pat. No. 4,324,564 discloses and claims a method of operating sorption beds (the Four Front method) having many significant advantages. In the context of this invention, chief among the advantages characteristic of the Four Front operational method is that significantly less heat is required for regeneration and that "waste" exiting the sorption bed during regeneration is emitted in a less massive, more concentrated pulse, having a lower average temperature.